Inductively coupled unidirectional taps



April 22, 1969 K. A. SlMONS INDUCTIVELY COUPLED UNIDIHECTIONAL TAPSFiled Aug. 24. 1966 OTHER LINES SUCH 14$ /N$3'4 VIA COUPLER-5 SUCH A55AT TORNEYJ United States Patent Int. Cl. H0lp /14 US. Cl. 333- 14 ClaimsABSTRACT OF THE DISCLOSURE A directional coupler for inductivelycoupling CATV subscribe drop lines or the like to a coaxial feedercable. A coil is disposed adjacent the center conductor of the feedercable; but insulated therefrom. One side of the coil is connected to thecenter conductor of the drop line, and to a back-match resistor for thedrop line, the other side to the outer conductor of the drop line. Theback-match impedance is connected to the secondary of a transformer theprimary of which is connected across the feeder cable.

This invention relates to transmission line systems and, moreparticularly, to coupling arrangements between a main transmission lineor feeder line and one or more other transmission lines such as droplines so arranged that such other transmission lines (or drop lines) maybe added to the main or feeder line whenever desired to receive signalsfrom the latter line, without cutting the feeder line or interruptingthe normal flow of signals over the main or feeder line while in theprocess of adding such other drop lines to the feeder line.

In present transmission systems used for community antenna televisionsystems, known as CATV systems, directional couplers have been used toconnect the feeder line, usually in the form ofa coaxial cable, to oneor more drop lines which are also coaxial cables and are expected toreceive programs of TV and the accompanying signals. In suchsystems,'the structure of the directional coupler has made it necessaryto physically cut or open the feeder cable so as to insert in serieswith the central conductor of the cable either one or more coils of oneor more transformers,'or 'one or more other elements used to form adirectional coupler.

The physical cutting or opening of the feeder line or cable during theinstallation and the necessary testing procedures to check theoperations introduced an undesired interruption in program transmissionover the feeder line to all of the other drop lines connected to thefeeder line and this generzfily involved a fairly substantial revenueloss to the proprietors of the system and an inconvenience to thevarious subscribers. A former arrangement, which has been used fairlywidely as a directionalcoupler for this purpose, is shown and describedin applicants Patent 3,048,798, issued Aug. 7, 1962, entitledDirectional Coupler.

An object of the present invention is to provide an improved arrangementfor coupling directionally a feeder line to one or more drop lines sothat there will be no service interruption whatever in the normaltransmission of signals or other energy via the feeder line to any orall other drop lines while the system is undergoing changes during theinstallation in coupling additional drop lines to the feeder line or inremoving drop lines from the feeder line. In the arrangement accordingto this invention, the drops previously connected to the feeder linewill be continuously fed all of the signals or energy from the feederline substantially without inter- 3,440,571 Patented Apr. 22, 1969ruption or distortion during the entire process of adding the new dropsor of removing some of the drops. I

Another object of the. invention is to provide a unique unidirectionalcoupling arrangement which comprises very few components and isrelatively inexpensive to manufacture and install. The installation maybe performed substantially without interfering with normal andcontinuous transmission over the feeder line and without any appreciabledegrading of the service between the feeder line and the variousassociated subscribers loops. Thus, in the arrangement of thisinvention, the feeder line..will serve as a continuous reservoir ofsignal energy to be tapped unidirectionally by additional drop lines asmay be desired, and hence drop lines may be added to or removed from thefeeder line without in any way affecting the loops or drops previouslyconnected to the feeder line.

Another object of this invention is to provide an arrangernent forcoupling a feeder TV line to a plurality of TV drop lines so that TV(and related) signals may flow freely in the forward direction along thefeeder line to the drop lines with a predetermined attenuation, but suchsignals may flow in the opposite direction along the feeder line to thesame drop lines with a very much higher attenuation. Thisunidirectionality is especially important in color TV systems, wherereflections or o'iher'disturbing or. unwanted signals betweensubscribers lines may severely distort the quality of the picture or ofthe accompanying sound effects to be reproduced.

Another object is to provide a directional coupler for adding aplurality of TV drop lines, one by one, to a feeder TV line, so designedthat there will be no appreciable.refiection from any of the pluralityof droplines to the feeder line, both during the installation proceduresand,after the coupling has been completed. The arrangement will bedesigned to avoid any breakage or interruption of the feeder line or ofthe circuit between the feederline and any and all other drop lines thatwere previously connected to the feeder line, and it is further designedalso to maintain continuous transmission over such other lines.

This invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing, in whichFIG. 1 illustrates schematically the novel coupling arrangement of thisinvention, and FIG. 2 shows a sectional view of a general arrangement ofone form of coil structure for coupling to a coaxial conductortransmission system of the type shown in FIG. 1.

FIG. 1 illustrates a main transmission line in the form of a coaxialcable of a familiar type which consists of a central conductor 1 and anouter shield 2, both of which are metallic and may be spaced from eachother by a dielectric which may be, for example, in the form of foaminsulation. The coaxial cable system 1-2 may be used, for example, in aCATV system for feeding the usual TV programs and accompanying soundsignals carried by the main or feeder line 1-2 to one or more droplines, one of which is shown for illustration in the form of a similarcoaxial cable consisting of a central conductor 3 and an outer shield 4which are also insulated from each other by a suitable dielectric. Thecoaxial cable 3-4 (together with other similar cables which need not beshown in the drawing) are designed to receive the signals carried by themain or feeder line 1-2 for supplying those signals to suitablereceiving equipments located in the homes or ofiices of subscriberswhere such signals may be reproduced. The coupling network, shownschematically within the dotted line boundary 5 representing a housing,includes a coil 6 mounted on a core 7 of magnetic material, atransformer 8 which may be an auto-transformer, and a resistor 9 servingas a matching impedance.

The core 7 upon which the coil 6 1S wound is inductively coupled to thecentral conductor 1 of the main or feeder line 1-2. There is no physicalor conductive connection whatever between any part of theelectromagnetic structure 6-7 and the central conductor 1. One terminalof coil 6 is connected to the central conductor 3 of the subscribersline 3-4 which is connected to the receiving equipment such as a TV set(not shown). The other terminal of coil 6 is connected in series withresistor 9 which may be considered to be a reverse terminating impedanceor a back-match for the feed system 3-4. Resistance 9 is in turnconnected to the tap 10 on the auto-transformer 8, the outer terminalsof which are physically connected between the central conductor 1 andthe outer conductors 2 and 4.

FIG. 2 illustrates, by way of example, a sectional 'view of one form ofthe electromagnetic structure 6-7 shown generally in FIG. 1. Sheath 2has been cut away so to accommodate the structure 6-7. The structure 6-7is embedded in suitable insulation 11 as shown. There is a slot s ininsulation 11 to enable the structure 6-7 to be moved over the centralconductor 1. The coil structure 6-7 may be held in place as shown withinslot s by conventional means familiar to those skilled in the art, suchas plugging the remainder of the open slot with insulation and taping itclosed. The positioning and spacing of the core on the central conductoris therefore predetermined, and the core is protected against theweather. This construction would insure against any conductiveconnection between any portion of the coil 6 with any portion of thecentral conductor 1, Hence, any vibration or movement of the overallcoupling structure would not affect the electrical coupling relationbetween the electromagnetic structure 6-7 with respect to the maintransmission line 1-2.

In operation, TV or other signals flowing over the main or feeder line1-2 from left to right, for example, will be fed to the drop cable 3-4so that the signals may be faithfully reproduced by receiving equipmentconnected at the distant end of the drop cable 3-4. At the same time thelevel of signals passing through the point of the coupling mechanism 5to other points further to the right along line 1-2 will be but minutelyaffected by the tapped connection to coaxial cable 3-4. This is due tothe fact that the drop cable receives only a minor portion of the signalenergy flowing along line 1-2. Hence, additional taps, which may besimilar to-the one shown and described as component 5 of FIG. 1, maylikewise be applied at other or more distant points along the maintransmission line 1-2, without amplification, for supplying othersimilar drop cable systems, so that subscribers sets connected theretomay likewise be arranged to reproduce the same signals faithfully.

As transmission takes place from left to right along the maintransmission line 1-2, the flow of current over the central conductor 1will cause voltage to be induced in coil 6 which, at any instant, willhave a predetermined polarity. At the same time, the auto-transformer 8,which is electrically connected to the central conductor 1 in awell-known manner, will develop a voltage between tap 10 and the outerconductors 2 and 4 which corresponds to the level of signals traversingthe inner conductor 1 of the feeder line. This voltage will remainsubstantially constant and of a predetermined magnitude and polarity.However, the voltage induced in coil 6 will, with predeterminedadjustments of the components of the coupling equipment 5, besubstantially equal in magnitude and have the same polarity as thatdeveloped by autotransformer 8. In other words, the coil 6 voltage andthe auto-transformer 8 voltage will be substantially equal and in phasewhen transmission is taking place from left to right along the feedercable 1-2. On the other hand, when transmission takes place from rightto left over the feederline l- 2, similar currents flowing in thecentral conductor 1 w ll nduce in coil 6 an equal voltage which will beopposite m polarity to that de eloped by coil 6 when transmission wastaking place normally, that is, from left to right along cable 1-2. a

As already observed, coil 6 is wound about core 7 in such a directionthat the resulting voltage applied thereby to the central conductor 3 ofthe drop coaxial cable 3-4 will have, for signals traveling from left toright, a polarity which is the same as thatof the voltage derived fromthe auto-transformer 8. The voltage across auto-transformer 8 will beapplied through tap 10 and resistor 9 to the same central conductor 3.Under these conditions, these voltages will be additive in effect andhence signals in sufficient amplitude will be transmitted from thefeeder line 1-2 to the drop line 3-4. The subscribers radio setconnected at the distant end of line 3-4 will receive, and will be ableto reproduce, those signals with good fidelity.

On the other hand, signals or other effects flowing in the oppositedirection, that is, from right to left, over the feeder line 1-2 willinduce a voltage in the structure of coil 6 which will be opposite tothat derived from auto transformer 8. Since these opposing voltages aremade equal in magnitude, none of those signals or other effects will berelayed to the drop line 3-4. The coil 6 voltage obviously may beadjusted to full equality with the opposing voltage of auto-transformer8 by any wellknown expedients, such as by moving or rotating coil 6 orby a variable slug in or adjacent to coil 6. Hence the directivity ofthe device may be optimized after installation.

Those skilled in the art will realize that in order for these opposingvoltages to be equal over the broad range of signals hereincontemplated, the response of said autotransformer and said coil must besubstantially flat for the TV spectrum.

This arrangement clearly discriminates against all signals or effectsflowing in the reverse direction over feeder line 1-2 and such effectsdo not reach the drop line 3-4 in sufficient magnitudes so as to distortthe desired signals. A

The voltage across the auto-transformer 8 will remain the same whatevermay be the direction of transmission along cable 1-2; that is, whetherfrom left to right or from right to left. Thus, when transmission occursin the feeder line 1-2 from right to left, the voltages generated bycoil 6 and auto-transformer 8 will be, in a proper design, substantiallyout of phase, whereupon such opposing voltages will substantiallynullify each other.

The arrangement of the inductive coil 6 and the tapped auto-transformer8 serves to reduce, and practically to eliminate, in properly designedequipment, the reflection or return ripple which would otherwise occurin a nonbalanced tap system and would introduce distortion into thesignals.

In a properly designed system, the transmission level at the receiverconnected to line 3-4 may be in the range of 10 to 50 db lower than thelevel of signals that may traverse the feeder line 1-2. Such acomparative signal level will be suitable for transmission over theauxiliary line when it is carrying a plurality of TV programs occupyinga broad band of the frequency spectrum, as, for example, the bandextending from the subaudible frequency range to the UHF range,including that part of the region extending from 54 mHz. to 216 mHz.

Resistor 9 and coil 6 are connected via tap 10 and a part of theauto-transformer 8, as shown, to the terminal of the central conductor 3of the drop line 3-4 adjacent to the feeder line 1-2. This resistor 9serves as a backmatch or terminating impedance for line 3-4. Thistimpedance is designed to match the normal characteristic impedance ofline 3-4, for example, 75 ohms. This backmatch, or the impedance ofresistor 9, will act to absorb reflections or other signals or effectsreturned by irregu= larities in line 3-4 or by signals.or other effectsgenerated or picked up by the subscribers set. As a conse quence of thedirectional properties of the coupler, miscellaneous signals or otherundesired effects traveling along line 1-2 in a reverse direction willnot be relayed to the drop lines connected to the line 1-2, Thesuppression of these undesirable voltages will be etfective throughoutthe frequency ranges of all signals tending to flow in line 1-2. Thecore 7 of the electromagnetic struc ture may be made, for example, offerrite material, but any other suitable core material may beappropriate for this arrangement, 1

The arrangement shown in FIG. 1 is a practical unidirectional coupler,that is, it serves to transmit signals from the feeder line 1-2 to thedrop line 3-4 when they arrive from one direction on line 1-2, and itdoes not transmit any substantial amount of energy when signals arrivefrom the other direction on line 1-2, This unidirectional feature isaccomplished without any breakage or interruption of the feeder line atany time. Thus, subscribers loops previously connected to the feederline 1-2 will be substantially unaffected as additional couplers, suchas 5, and their corresponding loops, are installed across feeder line1-2, Similarly, subscribers loops connected to the main line will besubstantially unaffected when specified unidirectional couplers, such as5, and their corresponding loops are removed from the feeder line. Byavoiding breakage or interruption the main transmission line, servicecontinuities are maintained and substantial savings are incurred.

'It is apparent that in the arrangement of this invention, compensatingcomponents such as condensers or usual way as would be apparent to thoseskilled in the other reactive or resistive components may be used in theart. While this invention has been shown and described with reference toa CATV system and to certain frequency ranges, the features 'of thisinvention are equally applicable to transmission systems for other typesof signals and for other frequency ranges,

While this invention has been shown and described in certain particulareriibodiments merely for illustration, other embodiments will beapparent to those skilled in the art. Hence, the invention is to betreated as limited only by the scope of the appended claims What isclaimed is:

1. Apparatus for coupling a"-subscribers coaxial cable to a main coaxialcable carrying currents corresponding to TV programs to be transmittedto remote points, the coupling to be effected without breaking thecircuit of the main cable and without interfering with the continuity ofservice to the remote points, comprising a coil structure, which isinductively coupled to the main cable and produces voltages continuouslyproportional to, and in a predetermined phase relation to the currentsin the main cable, and atransformer connected between the center andouter conductors of the main cable and producing voltages proportionalto the voltages across the main cable and in a predetermined phaserelation to such voltages, and means for combining the two voltagesproduced and applying them to the subscribers cable.

2. Apparatus according to claim 1 wherein said inductively coupled coilis adapted to fit within a slot cut into the sheath of the main cable.

3. Apparatus according to claim 1, such apparatus also including animpedance matching the impedance of the subscribers cable and connectedat the coupler end thereof.

4. Apparatus according to claim 3 including a housing for enclosing bothcoil structures and the terminating impedance, said housing beingpositioned at the point where the two cables meet,

5. A coupler for coupling two coaxial cables to each other so as totransmit unidirectionally between said cables a broad band of signals,such as those corresponding to television programs, said coupler beinginstallable without breaking the circuit of either cable, comprisingmeans for deriving from the signals traversing one of the cables a firstvoltage corresponding to the currents flowing in said cable and a secondvoltage equivalent to that across said cable, and means for combiningsaid voltages so as to be additive for signals travelling in the normaldirec* tion on said cable and subtractive for signals travelling in theother direction, the first voltage deriving means consisting of a coilwound upon a magnetic core and positioned in inductive relation to thecentral conductor of the cable with which it is directly associated.

6. A coupler for coupling first and second coaxial cables for theunidirectional transmission of signals corresponding to televisionprograms or like signals between said cables, said coupler beinginstallable without breaking the circuit of the first cable, comprisingmeans for inductively generating a first voltage which corresponds tothe signal currents flowing over the first cable, means for generating asecond voltage which corresponds to the voltage across said first cable,and means for applying both voltages to the second cable, so as to beadditive when the signals are transmitted in one direction over saidfirst cable andbeing subtractive when the signals over said first cableare reversed in direction, the first voltage generating means consistingof a single coil mounted 'on a magnetic core which is inductivelyrelated to the central conductor of the first cable but has noconductive connection to said central conductor.

7. An arrangement corresponding to claim 6 which also includes aterminating impedance for the second cable, said terminating impedancebeing connected to the second cable at a point adjacent to the firstvoltage generating means.

8. An arrangement in accordance with claim 6 in which the first voltagegenerating means is enclosed within a housing positioned around thesheath of the first cable.

9. A coupler for coupling a first coaxial cable to a second coaxialcable so as to transmit unidirectionally a broad band of signals, suchas television signals, from the first cable to the second cable, withoutbreaking the circuit of the first cable, comprising a coil mounted on amagnetic core and disposed within a slot cut therefor in the dielectricof the first coaxial cable, and responsive solely to currents flowingserially over the first cable, said coil having no conductive connectionto the central conductor of the first cable, a second coil connectedacross the first cable and responsive to the voltages across the firstcable and having no capacitive or inductive coupling therewith, saidcoils being so coupled to the first coaxial cable and connected theretoand to each other and to the second cable so that the voltages of thetwo coils arecombined and applied to the second cable, being additiveduring signal transmission over the first cable in one direction and inopposition to one another during signal transmission over the firstcable in the opposite direction.

10. An arrangement according to claim 9 which includes a terminatingimpedance connected across the second cable at a point where both cablesmeet, said terminating impedance serving to substantially prevent anyreturn of energy from the second cable to the first cable.

11. Apparatus for coupling a subscribers coaxial cable to a main coaxialcable carrying currents corresponding -to TV programs to be transmittedto remote points, the

coupling to be effected without breaking the circuit of the main cableand without interfering substantially with the continuity of service tothe remote points comprising: means inductively coupled to said maincable for producing a first voltage proportional to the current flow ata selected point within said main cable, means connected across saidmain cable for producing a second voltage proportional to the voltageacross said main cable, means for combining said first and secondvoltages so as to be additive when the current in said main cable isflowing in the normal direction, and subtractive when the current isflowing back through said main cable, and means for applying the sum ofsaid voltages to the subscriber cable, wherein said inductive meanscomprises a coil so disposed about the center conductor of said maincoaxial cable as to be closely coupled thereto, and one side of whichconnects to the center conductor of said subscriber cable.

12. The device of claim 11, further comprised of means to block currentfiow within said subscriber cable in opposition to that induced withinsaid coil by the normal signal flow in said main cable, including animpedance matching that of said drop line and connected between theother side of said coil and the secondary of said transformer 13, Thedevice of claim 12 wherein said coil is so positioned with respect tothe center conductor of said main 8 1%. The device of claim 13 whereinsaid means for producing a second voltage comprises a transformer thesecondary winding of which connects to the other side of said coil.

References Cited UNITED STATES FATENTS 2,531,438 11/1950 Jones 343-s52 xUS. Cl, X.R.

